DE102013100400A1 - Reactant control method for a fuel cell system in idling stop operation - Google Patents

Abstract

A system and method for controlling the reactants in anode and cathode regions of a fuel cell stack while the fuel cell stack is in a standby or idle stop mode. The method includes identifying a voltage setpoint for an average voltage of the fuel cells in the fuel cell stack or a total stack voltage that is a minimum voltage acceptable for idle-stop operation. The actual cell voltage average or actual stack voltage is compared to the voltage setpoint to produce a voltage regulation difference. The voltage regulation difference is provided to a controller that supplies hydrogen gas to an anode region of the stack to increase the anode region pressure, which reduces the voltage regulation difference if the voltage is above the voltage setpoint and / or supplies more cathode air to the cathode region of the fuel cell stack if the voltage regulation difference is below the Voltage setpoint is.

Description

BACKGROUND OF THE INVENTION

1. Field of the invention

This invention relates generally to a system and method for controlling the reactants within a fuel cell stack while the stack is in a stand-by mode or an idle-stop mode, and more particularly to a system and method for controlling the reactants within a fuel cell stack while the stack is in a stand-by mode or an idle-stop mode, the method controlling the anode region pressure by supplying hydrogen to the anode region of the stack and / or controlling the cathode air flow to the cathode region of the stack.

2. Discussion of the Related Art

Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electrochemical device including an anode and a cathode, between which an electrolyte is disposed. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free hydrogen protons and electrons. The hydrogen protons pass through the electrolyte to the cathode. The hydrogen protons react with the oxygen and electrons in the cathode, producing water. The electrons can not pass through the electrolyte from the anode. Accordingly, they are passed over a load to perform work before they reach the cathode.

Proton Exchange Membrane Fuel Cells (PEMFC) are a popular fuel cell for vehicles. A PEMFC generally includes a solid polymer electrolyte proton conductive membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalyst particles, usually platinum (Pt) dispersed on carbon particles and mixed with an ionomer. The catalyst mixture is applied to opposite sides of the membrane. The combination of the anode catalyst mixture, the cathode catalyst mixture and the membrane define a membrane electrode assembly (MEA). MEAs are relatively expensive to manufacture and require certain conditions for effective operation.

Typically, multiple fuel cells are combined into a fuel cell stack to generate the desired performance. For example, a typical fuel cell stack for a vehicle 200 or have more stacked fuel cells. The fuel cell stack receives a cathode input gas, typically with an air flow passing through the stack by means of a compressor. Not all of the oxygen is consumed by the stack, and some of the air is discharged as the cathode exhaust, and the cathode exhaust may include water as a stack waste product. The fuel cell stack also receives an anode hydrogen input gas that flows into the anode side of the stack. The stack further includes flow channels through which a cooling fluid flows.

A fuel cell stack typically has a series of bipolar plates disposed in the stack between the plurality of MEAs, with the bipolar plates and the MEAs disposed between two end plates. The bipolar plates include an anode side and a cathode side to adjacent fuel cells in the stack. Anode gas flow channels are provided on the anode region of the bipolar plates that allow the anode reaction gas to flow to the respective MEA. Cathode gas flow channels are provided on the cathode region of the bipolar plates that allow the cathode reaction gas to flow to the respective MEA. One end plate includes anode gas flow channels and the other end plate includes cathode gas flow channels. The bipolar plates and end plates are made of a conductive material such as stainless steel or a conductive composite material. The end plates divert the electricity generated by the fuel cells out of the stack. The bipolar plates further include flow channels through which a coolant flows.

During normal operation of a fuel cell system (FCS), some parasitic losses occur which reduce system efficiency. These losses include diffusion of hydrogen from the anode region of the stack to the cathode region of the stack, electrical shorts, and power consumption of accessories such as pumps, the compressor, etc. If no electrical power is desired from the fuel cell stack, the parasitic losses can be reduced by reducing the Flow of reactants are reduced to the fuel cell system.

There are cases where the fuel cell vehicle requires very little power, for example, when the fuel cell vehicle is stopped at a red traffic light. Providing a normal reactant flow to the fuel cell stack is generally wasteful in these situations because the reactant permeation and electrical load demand from the installed components can be very significant. It is generally desirable to reduce stack output power and power consumption during these idle conditions to improve fuel consumption of the system fuel.

For these and possibly other fuel cell system operating conditions, it may be desirable to put the system in a stand-by mode or an idle-stop mode, with the system receiving little or no power, the amount of fuel consumed is minimal and the system can quickly ramp up from stand-by to increase system efficiency and reduce system degradation. U.S. Patent Application Serial Number 12 / 723,261, entitled "Stand-by Operation to Optimize Lifetime Efficiency of a Fuel Cell Vehicle Application" filed March 12, 2010, by the assignee of this application and incorporated herein by reference, discloses a known method of relocating a vehicle fuel cell system in a stand-by mode to save fuel.

Since the reactant flows are reduced during stand-by or idle-stop operation and the concentration of reactants in the anode and cathode regions decreases, undesirable conditions may develop in the fuel cell stack. For example, hydrogen migrates through the membranes and accumulates in the cathode region when there is no air flow to the cathode region of the stack where a hydrogen / nitrogen / water mixture forms. Then, when power is required from the fuel cell system, it may be necessary for the hydrogen-rich gas in the cathode region to be mixed with diluents to avoid excessive hydrogen in the vehicle exhaust. This dilution process slows down the re-start of the fuel cell system and can cause undesirable power drops.

In addition, maintaining a hydrogen-rich concentration in the anode area throughout stand-by operation is also important. Without sufficient hydrogen delivery to the anode region, oxygen present in the cathode region may migrate into the anode region. Significant local concentrations of both oxygen and hydrogen in various regions of the anode region can cause a hydrogen / air front which can cause significant carbon corrosion of the cathode electrode, which is well known to those skilled in the art.

In order to control the oxygen enrichment in the anode region or the hydrogen enrichment in the cathode region, the precise control of the air and hydrogen reactants of the fuel cell stack is critical.

SUMMARY OF THE INVENTION

In accordance with the teachings of the present invention, a system and method for controlling the reactants in anode and cathode regions of a fuel cell stack while the fuel cell stack is in a stand-by mode or an idle-stop mode are disclosed. The method includes identifying a voltage setpoint for a mean voltage of the fuel cells in the fuel cell stack or a total stack voltage that is a minimum voltage acceptable for idle-stop operation. The actual cell voltage average or actual stack voltage is compared to the voltage setpoint to produce a voltage regulation difference. The voltage regulation difference is provided to a controller which supplies hydrogen gas to the anode region of the stack to increase the anode region pressure if the voltage is above the voltage setpoint, which reduces the voltage regulation difference and / or supplies more cathode air to the cathode region of the fuel cell stack, if Voltage control difference is below the voltage setpoint.

Further features of the present invention will become apparent from the following description and the appended claims, taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE FIGURES

1 is a simple schematic block diagram of a fuel cell system;

2 Fig. 12 is a graph plotting time on the horizontal axis and the voltage regulation difference and the anode pressure at various locations on the vertical axis showing a relationship between the anode pressure and the voltage regulation difference;

3 FIG. 10 is a control system that controls the anode area pressure or the cathode area airflow when a fuel cell stack is in an idle-stop mode; FIG. and

4 is a graph in which the time on the horizontal axis and the voltage regulation difference and the cathode flux are plotted at various locations on the vertical axis showing a relationship between the voltage regulation difference and the cathode flux.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The following discussion of embodiments of the invention directed to a system and method for controlling the reactants within a fuel cell stack when the stack is in a stand-by or idle-stop mode is merely exemplary in nature and none Way to limit the invention or its applications or uses. For example, the present invention finds a particular application in a fuel cell system on a vehicle. However, it will be readily understood by those skilled in the art that the system and method of the invention may have other uses.

1 is a schematic block diagram of a fuel cell system 10 with a fuel cell stack 12 , A compressor 14 provides an air flow to the cathode region of the fuel cell stack 12 on a cathode input line 16 for example, a water vapor transfer unit 18 (WVT), which humidifies the cathode inlet air. A cathode exhaust gas is removed from the stack 12 on a cathode exhaust gas line 20 left out the cathode exhaust gas to the WVT unit 18 conducts to provide the moisture for humidifying the cathode input air. A pressure relief valve 38 is in the exhaust pipe 20 which is regulated to the pressure within the cathode region of the fuel cell stack 12 to regulate. A bypass line 22 is about the WVT unit 18 arranged around a portion of the cathode exhaust gas or the entire cathode exhaust gas around the WVT unit 18 around pass. In an alternative embodiment, the bypass line 22 be an intake bypass. A bypass valve 24 is in the bypass line 22 This is controlled to selectively control the cathode exhaust gas through the WVT unit 18 or around the WVT unit 18 to provide the desired amount of moisture for the cathode inlet air. A bypass line 42 is around the fuel cell stack 12 arranged around and a proportional valve 40 is in the bypass line 42 arranged to regulate how much of the air flow from the compressor 14 through the pile 12 and how much around the pile 12 is led around.

The fuel cell stack 12 receives hydrogen from a hydrogen source 26 , the hydrogen gas in the anode region of the fuel cell stack 12 on an anode input line 28 by means of an injector 30 injected. An anode exhaust gas is removed from the fuel cell stack 12 on a recirculation line 32 which recirculates the anode exhaust back to the anode inlet by sending it to the injector 30 which can be operated as an injector / ejector, which is well known to those skilled in the art. A suitable example of an injector / ejector is in the US 7,320,840 entitled "Combination of an Injector Ejector for Fuel Cell Systems" filed by the assignee of this application and incorporated herein by reference. As is well known in the art, nitrogen accumulates in the anode region of the stack 12 which reduces the concentration of hydrogen there and the performance of the system 10 impaired. A bleed valve 34 is in the recirculation line 32 arranged to periodically vent the exhaust gas to remove nitrogen from the anode subsystem. The deaerated anode exhaust gas is via the vent line 36 to the cathode exhaust gas line 20 guided.

When the pile 12 go into stand-by or idle-stop operation, reactants are in the anode and cathode regions of the stack 12 available. Typically, most of the oxygen in the cathode is consumed slowly, either by a low stack load or by membrane permeation, along with limited or no cathode air delivery. However, as noted above, hydrogen and oxygen continue to diffuse through the MEA and the lines as soon as the standby or idle stop operation is initiated. One through the fuel cells in the stack 12 The voltage generated indicates the presence of oxygen in the cathode region of the stack 12 and hydrogen in the anode region of the stack 12 The presence of oxygen in the cathode region limits the accumulation of hydrogen in the cathode region. Too much voltage can be an indication that there is enough oxygen in the cathode area to diffuse to the anode area and cause a lifetime problem there. Thus, a setpoint voltage can be determined that determines the optimum reactant concentration in the anode region and the cathode region of the stack 12 during standby or idle stop operation. Increasing the anode region pressure by supplying hydrogen to the anode region will increase the permeation rate of hydrogen to the cathode region. Hydrogen that has migrated to the cathode region will react with the oxygen in the cathode region and reduce the voltage.

An embodiment of the present invention proposes to define a voltage setpoint (VSG) that provides a setpoint voltage for the fuel cell stack that discussed above Problems addressed when the stack 12 in a standby mode or an idle stop mode. The voltage setpoint may be any low voltage value, for example 100 mV, suitable for a stand-by operating voltage for the cells. In one embodiment, this is an average voltage of the cells used as the voltage setpoint. In another embodiment, the total stack voltage is used as the voltage setpoint.

2 Fig. 12 is a graph plotted time on the horizontal axis, voltage regulation difference on the lower end of the vertical axis, and anode pressure on the upper end of the vertical axis. The curve 50 represents the voltage setpoint and the curve 52 represents the calculated or measured voltage, for example the average cell voltage. The curve 54 shows an anode pressure change within the anode area of the stack 12 , which is used to calculate the voltage regulation difference by injecting hydrogen into the anode region of the stack 12 to settle what will be discussed in detail below. Hydrogen leading to the cathode area of the stack 12 migrates, will react with oxygen and reduce the cell voltage, which causes the anode area pressure to drop and the voltage regulation difference to increase. Supply of hydrogen to the anode area of the stack 12 causes the anode region pressure to increase, causing hydrogen to migrate to the cathode region, which reduces the voltage regulation difference. Thus, it can be shown that by controlling the anode area pressure, for example, by injecting hydrogen into the anode area, corrections can be made to the voltage regulation difference.

3 is a block diagram of a control system 60 that can be used to control the stack voltage or the cell average voltage during idle-stop operation. The voltage setpoint is removed from the box 62 a comparator 64 provided and the cell average voltage or the stack voltage is from a voltmeter 66 or any other suitable device to the comparator 64 made available. A typical fuel cell system will monitor both the stack voltage and the cell average voltage for various reasons, and many circuits and algorithms are known in the art for these purposes. The difference between the voltage setpoint and the actual voltage is the voltage regulation difference that is in the curve 52 which is a controller 68 is made available. Depending on whether the voltage regulation difference above or below the setpoint, by the curve 50 is determined, it is determined whether the controller 68 will cause hydrogen in the stack 12 what is injected in the box 70 is shown. As mentioned above, the controller becomes 68 then cause hydrogen in the stack 12 is injected, which causes the voltage regulation difference to drop with a dip. The controller 68 is the injection of hydrogen at some point, which can cause the voltage regulation difference over the setpoint curve 50 shoot out, stop. The hydrogen consumption within the stack 12 will cause the voltage regulation difference to move towards the setpoint curve 50 moved back, with the controller 68 again the injector 30 causes hydrogen in the stack 12 to inject.

In an alternative embodiment, the control system 60 be used to control the flow of air to the cathode area of the stack 12 to regulate and reduce the voltage regulation difference. The feeding of air to the cathode area of the stack 12 will displace and / or use up the hydrogen in the cathode area. As oxygen is absorbed on the surfaces of the cathode electrode, the cell voltage will increase. In other words, there is not enough oxygen in the cathode area of the stack 12 when the voltage is below the setpoint, with hydrogen diffusing from the anode region to the cathode region faster than the amount of oxygen entering the cathode region.

The condition described above can be found in the 4 which is a graph in which the time is plotted on the horizontal axis and the voltage regulation difference on the vertical axis are the same voltage regulation difference and the same reference value curves 50 and 52 from the 2 shows. The vertical axis further includes a cathode flux curve 72 , the settings in the cathode air in the cathode region of the stack 12 in response to the voltage regulation difference.

As from the comparison of the curves 52 and 72 can be seen, the controller increases 68 the air in the cathode area of the stack 12 then if the voltage regulation difference is below the setpoint curve 50 lies or towards the setpoint curve 50 moved, and the controller 68 reduces or causes the reduction of cathode air in the cathode area of the stack 12 then, if the voltage regulation difference above the setpoint curve 50 lies. In this embodiment, the controller would 68 the speed of the compressor 14 increase to provide more air and / or the position of the bypass valve 40 to reduce the air bypass, and / or the position of the pressure relief valve 38 Adjust the flow of air from the stack 12 to reduce.

Alternatively, the regulatory system 60 both the amount of hydrogen for the anode and the air flow to the cathode simultaneously or as part of the same regulation regulate.

In alternative embodiments, parameters other than the voltage may be used to generate a control difference. For example, concentration sensors may be used to generate a concentration rule difference, wherein the control system 60 supply more gas to the anode region and / or more cathode air to the cathode region to cause the concentration to be adjusted to a concentration setpoint.

As will be well understood by those skilled in the art, various or some of the steps and methods discussed herein to describe the invention may be performed by a computer, processor, or other electronic computing device that manipulates and / or manipulates data using electrical phenomena transformed. These computers and electrical devices may include various volatile and / or nonvolatile memories including a fixed computer readable medium having an executable program thereon containing various codes or executable instructions executed by the computer or processor, the memory and / or the computer readable medium may include all forms and types of memory and other computer readable media.

The foregoing discussion shows and describes purely exemplary embodiments of the present invention. One skilled in the art will readily recognize from the discussion of the attached figures and claims that numerous changes, modifications and variations can be made therein without departing from the spirit and scope of the invention as defined by the following claims.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• US 7320840 [0020]

Claims (9)

A method of controlling one or more reactants in a fuel cell stack when the fuel cell stack is in a stand-by mode or an idle-stop mode, the method comprising: Identifying a parameter of the stack that changes during operation of the fuel cell stack; - identifying a setpoint for the parameter for the stand-by mode or for the idle-stop mode; - monitoring the parameter when the stack is in the stand-by mode or in the idle-stop mode; Comparing the setpoint with the parameter during standby or idle operation of the stack to produce a control difference; and Reducing the control difference by supplying hydrogen to an anode region of the fuel cell stack and / or air to a cathode region of the fuel cell stack. The method of claim 1, wherein identifying a parameter of the stack and identifying a setpoint includes identifying a stack voltage and a stack voltage setpoint. The method of claim 1, wherein identifying a parameter of the stack and identifying a setpoint includes identifying an average cell voltage of the fuel cells in the stack and an average cell voltage setpoint. The method of claim 1, wherein reducing the control difference by supplying hydrogen to an anode region of the fuel cell stack and / or air to a cathode region of the fuel cell stack includes supplying hydrogen to the anode region of the fuel cell stack. The method of claim 4, wherein supplying hydrogen to the anode region of the fuel cell stack includes injecting hydrogen into the anode region of the fuel cell stack using an injector. The method of claim 1, wherein reducing the control difference by supplying hydrogen to an anode region of the fuel cell stack and / or air to a cathode region of the fuel cell stack includes controlling a rotational speed of a compressor that delivers air to the cathode region of the fuel cell stack. The method of claim 1, wherein reducing the control difference by supplying hydrogen to an anode region of the fuel cell stack and / or air to a cathode region of the fuel cell stack includes controlling a position of a bypass valve in a bypass line that guides air around the fuel cell stack. The method of claim 1, wherein reducing the control difference by supplying hydrogen to an anode region of the fuel cell stack and / or air to a cathode region of the fuel cell stack includes controlling a position of a pressure relief valve in a cathode region exhaust gas line from the fuel cell stack. The method of claim 1, wherein the fuel cell stack is on a vehicle and the stand-by or idle-stop operation is initiated when the vehicle is stopped.

Images